1 5/8" headers vs 1 3/4" headers on a 347cid

Has anyone dyno'd the difference between 1 5/8 headers against 1 3/4" headers? I am running a 66 Fastback 347 cid with AFR 205 heads. AFR suggests using 1 3/4" headers but, if there isn't a significant difference in HP and torque I don't want to spend $600 - $700 for just for a marginal gain.

Thanks for any replies.

I am not really sure of the difference but I know of a guy with a 347 that runs a comp solid roller Victor jr intake, 750 carb, victor jr ported heads, 10.1 comp, with 1 5/8 hooker headers and the car makes 420RWHP runs 11.40s in the quarter N/A and on the spray he runs 10.60's. So from the looks of that combo it runs pretty dang hard. Now I am sure he would gain more going to a bigger header but that is some very respectable numbers in my book out of a N/A 347!! The car was actually in Hot Rod mag a while back.
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The difference is not justifiable...unless you're really willing to spend the cash for every last HP.
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hp cost $$$
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less then 10HP

I had some interesting header results on my 418 w/AFR205's

Bought 1 3/4" from Ford Powertrain. Made more power than stepped 1 7/8"-2" and full 2"....

I can't see that your 347 needs more header than what you have. What does make a difference is the port match. Most headers don't match the square (vs rectangular) port of the AFR head.
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Port match, plus you have to realize, what most headers do is shift the power curve forward or backward. Meaning, while peak numbers will drop slightly with smaller headers, average horsepower will remain similar, with a slight emphasis in lower numbers. A larger primary tube will shift the numbers higher, but again, average numbers will remain similar, with the emphasis being in the 'after peak' range...meaning it won't fall off as fast. Stepped tube headers are a nice add on, because they allow you to broaden BOTH areas of the curve, adding more average horsepower, but your peak won't change too very much.

We learned this with much trial and error on circle track cars. And there was a nice writeup in Engine Masters magazine recently. But for the 3/8 mile we'd use the small tubes (1/34 lol), for the half mile we'd use 2". This helped 'tune' our off the corner torque. Now we just run a stepped primary, and it does the job of switching headers for us! =)

Cris

<font size=-1>[ This Message was edited by: Is1BadFord on 3/9/06 11:38am ]</font>

Here's the article BadFord is talking about.....it'll give you some useful info:

http://www.popularhotrodding.com/eng...exh/index.html
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Primary length and collector design/length will play a bigger part in HP production on a given engine than tube diameter.

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On 2006-03-08 21:37, mstngjoe wrote:
Here's the article BadFord is talking about.....it'll give you some useful info:

http://www.popularhotrodding.com/eng...exh/index.html

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That's a different article than I meant, but that one is even more specific! I mean you can tick off the points of your exhaust system as you read that, and by the time you're done, you know what headers, collectors, and mufflers to use lol.

Nice link!

Cris